The invention relates to wiper plugs for cleaning of residual cement from inside tubular strings, carriers for such wiper plugs and a method for use of the wiper plugs to clean long sections of pipe wherein the wiper plugs and carriers are arranged in the pipe at different locations so that new wiper plugs can be deployed to clean remote sections of the pipe.
In the oil and gas industry, when a well is drilled a casing is put into the well and it needs to be secured by cementing the casing to the wellbore. The cement is introduced through the inner diameter of the pipe and after the cementing process is completed, the inner diameter of the pipe must be cleaned to remove any remaining cement therefrom.
Wiper plugs are thus used to displace cement from a tubular string that has just had cement pumped through its lower end and into a surrounding annular space to seal it. Typically, a cement shoe keeps the pumped cement from coming back from the annulus into the string through which it was delivered. Some systems deliver a wiper plug as a spacer before the cement is delivered and introduce another wiper to go behind the cement, but the second wiper is the one that does the work in wiping the cement from the annulus of the pipe string.
Wiper plugs generally comprise of a series of cone shaped structures parallel to each other generally made of a resilient material. They are held above a wellhead in an enclosure called a lubricator which in essence is a long tubular with valves near opposed ends. The lubricator allows the well to be isolated to load the wiper plug or plugs and then the loading valve to be closed and the drop valve to be opened to release the wiper plug into the wellbore string. The string generally has a sub for catching the wiper plug called a landing collar.
The wiping plug that is introduced into the pipe is pressurized to travel from the surface down to the toe wherein it cleans and pushes any residual cement to the toe valves. It is important that the cleaning process be sufficient to remove the residual cement so that it does not plug the toe valve during operation of the well.
Deploying wiper plugs into the wellbore to clean residual cement has been used for years and is very effective at cleaning up to depths of about 5,000 feet. After that distance, the wiper plug loses effectiveness as the rubber or elastomeric wiping flanges exhibit wear due to abrasion. The flanges actually become torn up due to the distance traveled and the amount of cement to be removed from the interior of the pipe strings. The use of longer or multiple wiper plugs can provide better cleaning action but the cleaning effectiveness of those plugs also becomes reduced when greater lengths of pipe strings need to be cleaned.
The present invention now provides a solution to this problem by providing wiper plugs that significantly enhance the effectiveness of cleaning of interior pipe strings over distances over 10,000 to as long as 25,000 feet or more.